brianl

Hi,
Can anyone provide this formula for me?
Thanks
BrianL

Steve Crook

It depends on the rate at which the film is projected, usually measured in feet per second (fps).
The most common projection rate for feature films is 24 fps.
Smaller formats like 16 or 8mm are sometimes run at different rates like 18fps.

Projection rate (fps) = Length of film (ft) / Time (secs)
Length of film (ft) = Projection rate (fps) * Time (secs)
Time (secs) = Length of film (ft) / Projection rate (fps)

Steve

Steve Crook

You're right - but it is still related to the rate that the film is projected at - it just needs different figures plugged into those equations.

Steve

Hugo

For standard 35mm film as used in most cinemas there are 16 frames per foot of film. If this is projected at the usual speed of 24 frames per second then 1.5 feet of film will go through the projector per second.
This works out at 90 ft of film per minute.
The BFI magazine SIGHT AND SOUND now, for some reason, quotes the length of films only in feet, therefore divide this figure by 90 to get the running time in minutes.

brianl

Thanks to you all for your helpful comments - they were much appreciated.
Hugo, would it be reasonably accurate to use 90ft of film per minute for commercial films produced in the 30's and 40's?
Brianl

Hugo

Yes Brian I am sure it would. With the advent of sound films in 1929 both camera and projection speeds had to be standardised. 24 frames per second was the speed that was chosen then and has been used ever since in the cinema.

(TV of course, for technical reasons, uses 25 frames per second which is why films on TV, video, DVD etc. are always 4% shorter than the cinema version)

sanndevil

Kind of! The concept of frames per second isn't really correct for television signals - the misunderstandings have occurred because a generation of computer users have got used to software packages such as Adobe Premier saying choose "25fps PAL or 30fps NTSC". But this is simplistic and basically incorrect.

TV signals in this country are interlaced on a 625 line screen at 50Hz. That means the picture is painted by an electron beam which paints the top row, then skips a row, then paints the next row, then skips a row. When it gets to the bottom, it starts again from the top row + 1, then row + 3, thus filling in the alternate rows. This painting happens 50 times a second (i.e. 50Hz), so basically the odd rows and even rows get refreshed 25 times each, so effectively the entire picture is refreshed 25 times a second. However, many people complain that there is flickering between the odd and even rows - a shortcoming of the interlaced system. There is no concept of 'frames' in this model - because somewhere in the picture a row is always being repainted.

When it comes to broadcasting 24fps films, someone realised that 25 refreshes a second with 50Hz is very close to 24fps, so yes, they are just broadcast 'as is' after being converted to digibeta tape (or whatever) which means they run slightly fast, and of course the sound is at a slightly higher pitch.

Hope that made sense!

ollie

Very interesting San, why dosn't a 100hz tv picture look twice as good in a direct comparison ? it's prob not that simple

cheers Ollie.

"Bullseye !!"